[heap] Only allow valid inner pointers when calling GcSafeFindCodeForInnerPointer.

Bug: chromium:800251
Change-Id: I9cf94a1e17ca12970b5660f0320c02151d7b79a3
Reviewed-on: https://chromium-review.googlesource.com/856637Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50445}

src/heap/heap.cc

@@ -6607,9 +6607,7 @@ Code* Heap::GcSafeFindCodeForInnerPointer(Address inner_pointer) {
     return GcSafeCastToCode(this, large_page->GetObject(), inner_pointer);
   }
 
-  if (!code_space()->Contains(inner_pointer)) {
-    return nullptr;
-  }
+  DCHECK(code_space()->Contains(inner_pointer));
 
   // Iterate through the page until we reach the end or find an object starting
   // after the inner pointer.